An X-ray unit intended in particular for medical use, which has a plurality of radiographic planes offset with respect to one another, is referred to as a multi-plane X-ray unit. In this situation, each radiographic plane is essentially formed by an—in particular two-dimensional—X-ray detector. In most cases, a separate X-ray source is moreover associated with each radiographic plane.
X-ray units having two radiographic planes—also referred to as bi-plane or dual-source units—are used today in particular in 3D imaging (computer tomography, rotational angiography, etc.) or for the purpose of instrument navigation accompanying operations. The two radiographic planes of such units are in part, with respect to a central ray perpendicular to the particular radiographic plane, arranged offset from one another by a fixed angle. In this situation, the two radiographic planes can generally be rotated together around a common isocenter. On the other hand, for instrument navigation in particular, units having two emitter/detector pairs capable of independent movement are used. In this case, each of these emitter/detector pairs is as a general rule mounted on a separate C-arm.
When using a multi-plane X-ray unit it is possible on the one hand to produce three-dimensional image data sets, in particular so-called tomograms, of an object under examination in a shorter time than in the presence of only one radiographic plane. For three-dimensional localization of objects in the radiographic area of the X-ray unit, in particular for the purpose of instrument localization and navigation, the capture of an individual X-ray image by using two radiographic planes is also already basically sufficient such that a rotation of the radiographic planes is not required.
Error-free evaluation of the image data delivered by a multi-plane X-ray unit and a precise localization does however require a precise knowledge of the orientation of the radiographic planes in the space. This positioning of the radiographic planes is determined as a rule through calibration of the multi-plane X-ray unit. Such a calibration is performed in particular during commissioning of the unit. In order to avoid a maladjustment of the unit during actual operation, or at least to recognize such maladjustment, it may additionally be necessary to repeat the calibration at regular intervals during operation of the unit. In this case one also speaks of recalibration. In the case of units having independently moveable radiographic planes, such as double C-arm units for example, the problem can exist that for reasons of design only one of the radiographic planes is capable of stable calibration. In the case of such units, the second radiographic plane often needs to be frequently recalibrated, sometimes with a considerable degree of adjustment.